The present invention relates to a stethoscope, more broadly sometimes referred to as a vital signs monitor, and more particularly to a stethoscope which has improved fidelity of reproduced body-generated signals.
For the purpose of this application, a stethoscope is defined as a device having a body signal transducing device which is placable on the body, coupling to the skin and serving as the pick-up for sounds generated in the body and transmitted through the flesh which, for the purpose of this application, is defined as the skin and underlying tissue. Since the stethoscope is frequently used to monitor sounds due to action of the heart, in which case the body signal transducing device is placed on the chest, the body signal transducing device is frequently referred to a chest piece, and is hereinafter referred to as such. The stethoscope as used in this application is considered in its broadest sense. The stethoscope, as it is defined in the present application, also has an output element which allows a user of the stethoscope to perceive the sounds being generated by the body. This output element can be provided by a variety of devices such as earphones, speakers, recording mediums (capable of being played back), visual monitors, etc., all hereinafter referred to as earphones. In the stethoscope, the chest piece and the earphones are operatively connected by a transmission medium which, for a classical mechanical stethoscope where the output of the chest piece is transmitted through a gaseous medium such as air, can be a tube containing a column of air. Alternatively, for an electronic stethoscope where the output of the chest piece is an electronic signal, the transmission medium can be composed of a variety of electronic components.
There are a variety of stethoscopes which have been developed. All of these are based on techniques for sensing the sound emitted from the body by monitoring the sound that radiates from the body and is transmitted through a confined volume of a fluid. Historically, the fluid employed in the stethoscopes was air, and attempts were made to mechanically amplify the sounds generated by the body. Sound, being a wave form of energy, is classically described in terms of its intensity, reported in decibels (dB) with larger values corresponding to greater intensity, and its frequency, reported in cycles per second or Hertz (Hz).
The stethoscope was invented by Rene Laennec in France in 1819. His stethoscope was a monaural device which was fashioned from wood and had an input opening and an associated sound chamber or so-called xe2x80x9csound accumulatorxe2x80x9d, which was modeled on the shape of a musical horn. A sound transmission conduit (also fabricated from wood) was a straight, smooth bore which coupled the sound chamber to the physician""s ear. A major obstacle when using Laennec""s monaural stethoscope was the tendency of unwanted sounds from the surrounding environment masking the desired sounds, since one ear was directly subject to sounds from the surrounding environment.
A binaural stethoscope was developed by George Camman in 1850 in an effort to overcome or, at least, mitigate the impact of the sounds from the surrounding environment thereby enhancing the ability of the user of the stethoscope to listen to the sounds generated by the heart and other body organs such as the lungs. This stethoscope had flexible tubes which transmitted sound to both ears of the user, reducing the user""s exposure to sounds from the surrounding environment.
Both of these stethoscopes employed open-ended sound chambers which, when pressed against the skin of the patient, provided a confined volume of air for transmitting sound waves from the body of the patient to the earpiece. A significant problem with such open-ended stethoscopes is the low intensity of the sound transmitted to the observer""s ear(s), due to loss in intensity of the sound as it crosses the interface between the flesh of the patient and the air-filled sound chamber of the chest piece. This signal loss is due to the differences in mechanical impedance of the flesh, which is substantially liquid, and the air employed in the chest piece. This difference in mechanical impedance between the flesh and the air causes only a small portion of the sound wave energy to be transferred from the flesh of the body to the air in the chest piece. As a result, the signal received is low in intensity, and may be difficult to detect or distinguish over ambient noise from the surrounding environment.
Another problem associated with these stethoscopes is that the frequency response of these devices is dependent on the pressure with which the chest piece is applied to the body of the patient. As the pressure applied to the chest piece increases, the chest piece causes distortion of the flesh. This distortion results in the flesh of the patient filling a portion of the chamber and altering the volume and pressure of air in the sound chamber, altering its mechanical performance in amplifying sounds. Thus, the characteristic response of the chest piece is dependent on technique (the touch of the user), so the sounds heard may differ for different users, making comparison of results problematic.
A significant advance in overcoming the problem of low intensity (dB) of the sound transmitted to the user of the stethoscope was made by Dr. R. C. M. Bowles (Massachusetts General Hospital) and was patented in 1901. His improvement to the stethoscope was the addition of a thin semi-rigid diaphragm attached to the chest piece and covering the opening to a conical-shaped sound chamber. This was found to significantly increase the intensity (dB) of sounds generated by the heartbeat and transmitted to the user, achieving such increase by selectively amplifying sounds in the frequency band centered near 90 Hz, the band associated with many of the sounds associated with the heart. The Bowles type stethoscope design is one of the most common currently used, and few significant design changes have been made since its invention.
While the Bowles stethoscope significantly increases the intensity of sounds in the frequency range of many of the sounds generated by the heart, the use of a diaphragm creates frequency-dependent distortion due to the natural vibrational frequencies of the diaphragm. This creates a response which is highly dependent on frequency, having significantly increased sensitivity to sounds in the frequency bands centered at about 90 Hz and 300 Hz, and greatly reduced sensitivity in the frequency band centered near 200 Hz and above about 500 Hz. Thus, while the Bowles type stethoscope offers a significant increase in the intensity (dB) of many of the sounds generated by the heart and other sounds of similar frequency heard by the user of the stethoscope, it reduces the intensity (dB) of transmitted sounds generated by the heart or other organs which are in the frequency bands of reduced sensitivity, thus rendering the resulting stethoscope unsuitable for the observation of many sounds which may be of interest to the user. For this reason, many modem stethoscopes employ a combination chest piece which includes both an open-ended sound chamber, either conical or spherical in shape, and a Bowles type diaphragm-covered conical-shaped sound chamber, enabling the user to select the chamber best suited for monitoring the frequencies of interest.
Furthermore, if the diaphragm of the chest piece receives sounds from the surrounding environment which are in the frequency band of increased sensitivity, these sounds are also amplified, thus tending to obscure the sounds monitored by the user of the stethoscope.
The use of a thin semi-rigid diaphragm also results in a frequency-dependent response of the diaphragm which varies with the pressure on the chest piece as it is engaged with the body. Thus, when a thin semi-rigid diaphragm is employed in a stethoscope, a variation in response of the stethoscope results from the diaphragm changing its natural harmonic frequency as it engages the body. This pressure-dependent variation makes the repeatability of the signals highly dependent on the technique of the user in a manner similar to that of open-chamber stethoscopes, as discussed earlier. The natural harmonic frequency of the thin, semi-rigid diaphragm also introduces transient frequency shifts which can further distort the sound perceived by the user. This is particularly a problem when monitoring impulsive sounds, such as heartbeats.
More recently, in an attempt to improve the quality of sound perceived by the user, electronic stethoscopes have been developed. In electronic stethoscopes the sound waves are converted into an electronic signal by a transducer, which is frequently positioned at the rear or apex of the chest piece sound chamber formed by the horn or cone. This electronic signal is usually amplified and, in some cases, is transmitted to signal conditioning circuits such as electronic filters and/or power amplifiers, which provide the conditioned signal to speakers that form the earpieces. The signal conditioning circuits, when employed, are intended to enhance the quality of the output signal by amplifying the signal intensity of the desired sounds (those sounds being generated by the organ being observed by the user of the stethoscope) and attenuating or filtering out some undesired sounds (from the environment and other organ-generated sounds).
The transducers employed in electronic stethoscopes can be of various types. A classical microphone which is designed to convert airborne sound pressure waves into an electronic signal can be employed for the transducer. The microphone can be placed in the vicinity of the apex of the horn or cone, where it experiences mechanically amplified sound waves, or can be placed in tubes which extend from the chest piece and connect to the earpieces to simulate the characteristic sound reception of a mechanical stethoscope.
While electronic stethoscopes can provide amplified signals, they still suffer from frequency-dependent distortion (particularly when a conventional diaphragm is employed), pressure-dependent distortion, and from sound generated in the environment which reaches the sound chamber of the chest piece. Pressure-dependent variation can be particularly problematic where electronic processing of the received sounds is desired, since the pressure applied by the user can significantly alter the characteristics of the sounds to be processed, making filtering of the noise component difficult. Furthermore, such variation can make electronic recognition and/or analysis of the desired signal component of the sound impractical.
Amplification of the signal in electronic stethoscopes frequently results in amplification of sound from the surrounding in which the stethoscope is operated as well. Because these stethoscopes employ air as the fluid medium transferring sound waves to the transducer, they frequently require substantial amplification to compensate for the signal loss caused by limited transfer of the sound wave energy from the flesh to the transducer. If the intensity (dB) of the sound from the surroundings in the unamplified sound transmitted through the sound chamber is sufficiently great as to mask the sound generated by the organ of interest, simply amplifying the electronic signal produced by the transducer does not allow the sound emanating from the organ to be monitored.
There have been various attempts to reduce exposure of the sound chamber to sounds from the surrounding environment by providing covers on the chest piece to prevent airborne sounds being transmitted through the sound chamber to the transducer. U.S. Pat. No. 4,995,473 and 5,578,799 teach two examples of such covers.
In addition to sounds generated by the surrounding environment being transmitted through the sound chamber, another problem has been that movement of the chest piece over the skin of the patient generates sounds which are subject to amplification. These sounds may not only obscure the sounds of interest, but also may be of sufficient intensity as to cause discomfort to the user.
An alternative approach to enhance the performance of an electronic stethoscope is to employ a confined volume of liquid to transmit the sounds from the body to the transducer. This technique is beneficial in that it reduces the signal loss due to mechanical impedance by employing a liquid rather than air to transmit the sound waves to the transducer. U.S. Pat. No. 3,130,275 teaches the use of a piezoelectric transducer in combination with a cylindrical cavity to transmit the body sounds, the liquid being confined in the cavity by a membrane. U.S. Pat. No. 3,076,870 teaches the use of a capacitance transducer in combination with a liquid confined between the capacitor and a membrane. While the ""870 patent indicates that the liquid has xe2x80x9cdampingxe2x80x9d capacity for attenuating xe2x80x9cunwanted external soundsxe2x80x9d, to a large degree the external sounds attenuated would be in the higher frequency range, greater than about 1,000 Hz. Thus, the liquid would not substantially reduce the lower frequency sound, which is often a large component of the unwanted external sound.
A further problem with all stethoscopes that has not previously been appreciated is that xe2x80x9cunwanted external soundsxe2x80x9d can also be indirectly transmitted though the membrane, since the body provides a conduction path for these xe2x80x9cunwanted external soundsxe2x80x9d. Even in liquid-filled chest pieces, it is felt that such conducted unwanted sounds would not be effectively attenuated, independent of their frequency. This latter source of unwanted external sounds is particularly a problem in monitoring patients in high-noise environments, such as during transport where vehicle noise due to sirens, aircraft engines, helicopter rotors, etc. can be sufficiently loud as to mask the desired signals from the body. Since such transport often occurs in emergency situations where monitoring is necessary to providing prompt diagnosis and treatment, this deficiency of existing devices is a serious limitation.
Thus, there is a need for a chest piece which provides signals with reduced noise, including reduction of noise transmitted through the body.
It is an object of the invention to detect sound (body signals) generated by an animate human body and transmit these signals to a transducer without distortion or other artifacts.
It is another object of the invention to reduce the ambient noise sensed by the transducer of a chest piece.
It is another object of the invention to provide a stethoscope with a xe2x80x9cflat frequency response curvexe2x80x9d, defined as a curve where perceived intensity (dB) of sounds is not strongly dependent on their frequency (Hz), and which avoids the introduction of resonant frequency bands.
It is another object of the invention to provide a stethoscope having reduced transient response to received sounds.
It is another object of the invention to provide a body signal transducing device with reduced sensitivity to noise which is transmitted through the body.
It is still another object of the invention to provide a body signal transducing device that is immune to noise due to slight movement on the skin surface.
It is a further object of the invention to provide a stethoscope where the response is not dependent on the pressure with which the sensor is applied to the skin.
It is yet another object of the invention to provide a mechanical waveguide for a stethoscope to reduce the intensity of sound waves that propagate along paths which are not normal the skin of the body under observation.
The present invention is a chest piece which is suitable for use in a stethoscope having, in addition to the chest piece, earpieces which are operably connected to the chest piece by a transmission medium.
In an elementary form, the chest piece has a housing having a cavity which acts as a sound chamber. The cavity has a cavity surface which is a surface of revolution generated by a parabola having a parabola apex, a parabola focal point, and a parabola axis. The parabola is rotated about the parabola axis to generate the cavity surface such that the cavity surface is formed as a paraboloid, having a paraboloid apex coincident with the parabola apex, a paraboloid axis coincident with the parabola axis, and a paraboloid focal point coincident with the parabola focal point. The cavity of the housing is further configured such that the paraboloid focal point resides in the cavity and the cavity is terminated by the paraboloid apex and a substantially planar opening which is substantially normal to the paraboloid axis.
A transducer is provided for converting received sound into an electronic signal. Means for positioning the transducer at or near the paraboloid focal point are provided. In one embodiment of the invention, a tube that pierces the paraboloid apex of the paraboloid can be employed as the means, while in another embodiment the transducer can be supported by a spider-leg support which is secured to the cavity surface can serve as the means.
The parabolic shape of the cavity surface acts to reflect sound waves transmitted normal to the substantially planar opening (those sound waves generated by the area of the body to which the chest piece is applied) to the transducer at the focal point, while reflecting sound waves which are substantially inclined to the substantially planar opening (both unwanted sound waves generated externally and transmitted through the body and unwanted sound waves generated in other areas of the body, collectively noise) away from the focal point, thereby reducing the intensity of the noise at the location of the transducer.
It is further preferred that a support structure be provided that extends across the substantially planar opening and is substantially transparent to sound. It is further preferred that the support structure be a substantially open support structure configured to provide a series of passages therethrough, where the passages extend parallel to the paraboloid axis. The series of passages thus form a mechanical wave guide which reflects sound waves that are not propagating substantially parallel to the paraboloid axis.
While the above described chest piece is effective in reducing the intensity of noise, it is preferred that a membrane be provided, which covers the substantial planar opening of the cavity to form to a closed cavity. Preferably, the membrane is fabricated from a compliant material which has mechanical properties similar to human skin. This match is preferably accomplished by coordinating the selection of size, thickness, and composition of the membrane such that the membrane is essentially acoustically transparent, having a first mode natural frequency which is well below that of the signals to be monitored. Having the first mode natural frequency so selected reduces cancellation of the sounds of interest due to the natural frequency of the membrane, as well as excitement of these frequencies by the sounds of interest which could create transient effects and effectively make the output of the chest piece time dependent. Preferably, to minimize these problems, the first mode natural frequency of the membrane is preferably less than about 20 Hz. An example of a suitable material for the membrane would be a high strength, low durometer, molded polyurethane elastomer compound. It is also preferred that the membrane have a high coefficient of friction, to minimize the likelihood of sliding on the skin surface which would introduce noise which might be sensed by the chest piece. The membrane is preferably held in position with a bezel which attaches to the housing.
It is also preferred that the membrane be configured so as to provide a concave surface when viewed from the focal point of the parabola, thus being convex when viewed by the user. The convex shape helps assure that, when the membrane is placed against the skin of a patient, no air pockets which might adversely affect the response are trapped between the membrane and the skin. When the cavity is gas filled, the membrane may be maintained in a convex configuration by maintaining the pressure in the cavity slightly above atmospheric pressure. Alternatively, when substantially open support structure is employed, it can be made convex in form to support the membrane. Again, such a support structure is preferably configured to also provide a mechanical wave guide.
Preferably, the cavity is filled with a non-gaseous fluid, which is substantially incompressible and which fills the chamber such that the resulting membrane has a convex surface and thus the volume of the fluid remains constant. As used herein, the term fluid includes both liquids and semi-solid colloidal solutions, hereinafter referred to as gels. It is further preferred that a gel be employed, to reduce the likelihood of leakage in the event that the membrane is punctured. A gel is also preferred so that the membrane need not support the fluid, thus allowing the membrane to be thinner and have a lower modulus of elasticity than would be required if the membrane needed to support the fluid.
When a liquid or gel is employed in the cavity as a sound transmission medium, it is selected to have physical characteristics similar to those of human flesh, which is typically similar to the properties of water, so that the acoustical impedance matches that of the flesh. It is further preferred that the sound transmission medium be equal to or slightly lower in mechanical impedance than the flesh. A silicone gel is preferred for having physical properties that are not strongly temperature dependant over a wide range of temperatures.
A variety of transducers can be employed such as capacitance transducers, piezoelectric transducers, microphones, and hydrophones; however, it is preferred that when the cavity is gas-filled, the transducer is a microphone, and in the case that the cavity is liquid or gel-filled, a hydrophone is preferably employed as the transducer. When a microphone is employed, the microphone is preferably an omnidirectional electret type (cardioid) microphone to respond to both direct sound waves and sound waves reflected from the parabolic cavity surface of the chest piece. When a hydrophone is employed, it is preferred that it be omnidirectional in its response.
When a transducer having a substantially planar sound receiving surface is employed as the transducer in the chest piece, it is preferred that the parabolic cavity surface be contoured such that the transducer, which is at or near the focal point of the parabola, is in close proximity to the substantially planar opening and that the transducer be mounted on a spider-leg support such that the substantially planar sound receiving surface has an unobstructed view of the apex of the paraboloid. It is further preferred that a supplemental transducer be provided, the transducer mounted at or near focal point of the paraboloid serving as a primary transducer. The supplemental transducer is mounted between the primary transducer and the membrane or, when a mechanical wave guide is provided, the supplemental transducer is mounted between the primary transducer and the mechanical wave guide. The supplemental transducer serves to receive sound blocked from reaching the cavity surface by the primary transducer. The supplemental transducer is preferably positioned in close proximity to the paraboloid focal point to minimize the phase difference between the sound received by the two transducers. Furthermore, when the supplemental transducer has a substantially planar sound receiving surface, this surface is positioned substantially normal to the paraboloid axis and facing the membrane and/or the mechanical wave guide.
To further improve the directional response of the chest piece having a liquid- or gel-filled sound cavity, it is also preferred that a mechanical wave guide such as discussed above be employed. The mechanical wave guide is positioned between the membrane and the transducer. While the mechanical wave guide can have a convex surface which supports the membrane, this support is not needed to maintain the convex contour of the membrane, since the liquid or gel maintains pressure against the membrane to maintain its convex profile. Since such support is not needed, it is preferred that mechanical wave guide be in close proximity to but spaced apart from the membrane. Having the mechanical wave guide so positioned reduces the influence of the mechanical wave guide on the movement of the membrane and allows the membrane to more readily comply to any irregularities of the flesh on which it is placed.
In all applications, the support structure/mechanical wave guide is preferably constructed from a rigid material such as aluminum or a semi-rigid plastic material (polycarbonate, ABS, or similar engineering grade plastic). The mechanical wave guide preferably has a thickness of at least about xe2x85x9 inch (3 mm) and has an array of parallel sound passages extending therethrough.
While the frictional nature of the membrane is preferably sufficient to prevent inadvertent motion of the chest piece across the skin of the patient under normal use, to further reduce the possibility of noise due to motion of the chest piece across the skin, it is preferred for the stethoscope to be activated by a pressure switch. The pressure switch responds to fluid pressure inside the cavity, and only allows the stethoscope to activate when the pressure is sufficient to assure that the membrane of the chest piece is firmly applied to the skin. This firm pressure, in combination with the frictional nature of the membrane, is sufficient to prevent any noise generated by motion of the chest piece across the skin. The pressure switch is preferably connected in series with a manual switch, and interrupts power to one or more of the transducer, the transmission medium, or the earphones.